1. Field of the Invention
The present invention relates to a chiral nematic liquid crystal composition, for which the temperature dependency of the natural pitch and the temperature dependency of the wavelength selective reflection are minimal and the low temperature storage stability is excellent in a bistable liquid crystal display element, and a liquid crystal display element comprising the same.
2. Description of Related Art
In bistable liquid crystal display elements, in order to induce a twisted orientation, a chiral nematic liquid crystal composition containing an optically active compound with a liquid crystal compounds are used, and it is already known that increasing the content C (weight %) of the optically active compound causes a reduction in the natural pitch P (μm), and that for content C values up to several dozen % by weight, the relationship whereby the product of P and C remains constant often applies. Accordingly, the helical twisting power: HTP (1/μm) is defined by the following formula:HTP=1/(P×0.01C)(wherein C (weight %) represents the amount added of the optically active compound, and P (μm) represents the natural pitch). This value is used as an evaluation parameter or the twisting power unique to that particular optically active compound.
In a chiral nematic liquid crystal, for a planar state in which the helical axis is perpendicular to the substrate, selective reflection based on Bragg reflection theory occurs. The wavelength of that selective reflection is represented by the following formula:λ=n×P(wherein, λ represents the wavelength selective reflection, n represents the average refractive index of the liquid crystal, and P represents the natural pitch).
In order to achieve bistability characteristics, the natural pitch must be less than approximately 3 μm, and in order to achieve a wavelength selective reflection in the visible light spectrum, the natural pitch must be no more than 0.5 μm, and consequently, an extremely large amount of the optically active compound must be added. However, addition of a large amount of optically active compound can cause problems, including a narrowing of the liquid crystal temperature range, and potential precipitation of the chiral compound during low temperature storage, leading to a deterioration in low temperature storage stability. The use of an optically active compound with a large HTP value is one method that is effective in reducing the amount of the optically active compound that must be added, and a compound represented by the formula (VI-e) shown below has been disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 63-51359 and Japanese Unexamined Patent Application, First Publication No. Hei 6-265899.
However, the above publications disclosed only that the temperature dependency of the natural pitch was negative when the compound was used in a STN liquid crystal composition, and examples of applications of the compound to bistable liquid crystal display elements using selective reflection are unknown.
On the other hand, because the natural pitch shows a temperature dependency, the natural pitch fluctuates with the temperature, and variations in temperature cause variations in factors such as the display quality and the operating voltage. As a result, liquid crystal compositions for which the temperature dependency of the natural pitch is small, and the temperature dependency of the wavelength selective reflection is also small, have been keenly sought.
A technique for reducing the temperature dependency of the natural pitch and the temperature dependency of the wavelength selective reflection by combining optically active compounds with left and right helical induction directions has been disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 55-38869. However, this technique is not particularly efficient since sections develop in which the helical twisting power cancel each other out, and because the amount of the optically active compound must be increased to cope with this problem, the liquid crystal temperature range narrows.
Another liquid crystal composition for reducing the temperature dependency of the natural pitch and the temperature dependency of the wavelength selective reflection, by combining an optically active compound for which the temperature dependency of the natural pitch is positive, and a compound for which the temperature dependency is negative, has been disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 7-258641. Specifically, a composition is disclosed that combines a compound with a positive temperature dependency represented by a formula (VI-a) shown below:
and a compound with a negative temperature dependency represented by a formula (VI-b) shown below.

Here, the description of the natural pitch temperature dependency as positive means that as the temperature rises the pitch increases (stretches). In contrast, a negative natural pitch temperature dependency means that as the temperature rises the pitch decreases (shrinks). However at a temperature of 20° C., the HTP value of the compound represented by the formula (VI-a) is from 5 to 6, and the HTP value of the compound represented by the formula (VI-b) is from 1 to 2, which is extremely small. In order to achieve a bistable liquid crystal using these optically active compounds, approximately 80% by weight of the optically active compounds would need to be added. Addition of this type of extremely large amount of optically active compounds is impossible from a practical standpoint, and even if it were possible, it is clear that the liquid crystal temperature range would be extremely narrow.
In addition, another method for reducing the temperature dependency of the natural pitch, comprising a liquid crystal composition combining an optically active compound represented by a formula (VI-c) shown below:
and an optically active compound represented by a formula (VI-d) shown below:
has been disclosed in PCT International Publication No. WO97/29167, and the temperature dependent variation in pitch are reported as 0.07% per 1° C. However, the liquid crystal composition disclosed in PCT International Publication No. WO97/29167 is a STN liquid crystal composition, and in a composition capable of selective reflection, the wavelength selective reflection varies from 560 nm to 580 nm, namely from green to yellow, for a temperature variation from 0 to 50° C., making the composition unsuitable for practical use. Furthermore, a liquid crystal composition containing a large amount of the optically active compounds disclosed in PCT International Publication No. WO97/29167 in order to cause selective reflection also shows problems associated with low temperature storage stability.
In addition, a composition combining an optically active compound (product name: R-1011) represented by a formula (VI-f) shown below:
and an optically active compound (product name: R-811) represented by a formula (VI-g) shown below:
has been disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 11-323338 and Japanese Unexamined Patent Application, First Publication No. Hei 11-305187. This combination of optically active compounds shows a small natural pitch temperature dependency, and is widely used as a liquid crystal composition with selective reflection. However, the optically active compound R-1011 shows poor solubility, and liquid crystal compositions using this compound tend to have problems associated with low temperature storage stability.
As a result, a chiral nematic liquid crystal composition, for which the temperature dependency of the natural pitch and the temperature dependency of the wavelength selective reflection are minimal and the low temperature storage stability is excellent, together with a chiral nematic liquid crystal composition with a broad liquid crystal temperature range, have been keenly sought.